Lithium Pharmacology - Mechanism of Action

Mechanism of Action

Unlike other psychoactive drugs, Li+ typically produces no obvious psychotropic effects (such as euphoria) in normal individuals at therapeutic concentrations. Li+ possibly produces its effects by interacting with the transport of monovalent or divalent cations in neurons. However, because it is a poor substrate at the sodium pump, it cannot maintain a membrane potential and only sustains a small gradient across biological membranes. Li+ is similar enough to Na+ that under experimental conditions, it can replace Na+ for production of a single action potential in neurons.

Recent research suggests three different mechanisms which may or may not act together to deliver the mood-stabilizing effect of this ion. The excitatory neurotransmitter glutamate could be involved in the effect of lithium as other mood stabilizers, such as valproate and lamotrigine, exert influence over glutamate, suggesting a possible biological explanation for mania. The other mechanisms by which lithium might help to regulate mood include the alteration of gene expression.

Lithium may also increase the release of serotonin by neurons in the brain. In vitro studies performed on serotonergic neurons from rat raphe nuclei have shown that when these neurons are treated with lithium, serotonin release is enhanced during a depolarization compared to no lithium treatment and the same depolarization.

An unrelated mechanism of action has been proposed in which lithium deactivates the GSK3β enzyme. This enzyme normally phosphorylates the Rev-Erbα transcription factor protein stabilizing it against degradation. Rev-Erbα in turn represses BMAL1, a component of the circadian clock. Hence, lithium by inhibiting GSK3β causes the degradation of Rev-Erbα and increases the expression of BMAL which dampens the circadian clock Through this mechanism, lithium is able to block the resetting of the "master clock" inside the brain; as a result, the body's natural cycle is disrupted. When the cycle is disrupted, the routine schedules of many functions (metabolism, sleep, body temperature) are disturbed. Lithium may thus restore normal brain function after it is disrupted in some people.

Another mechanism proposed in 2007 is that lithium may interact with nitric oxide (NO) signalling pathway in the central nervous system, which plays a crucial role in the neural plasticity. The NO system could be involved in the antidepressant effect of lithium in the Porsolt forced swimming test in mice. It was also reported that NMDA receptor blockage augments antidepressant-like effects of lithium in the mouse forced swimming test, indicating the possible involvement of NMDA receptor/NO signaling in the action of lithium in this animal model of learned helplessness.

Lithium treatment has been found to inhibit the enzyme inositol monophosphatase, leading to higher levels of inositol triphosphate. This effect was enhanced further with an inositol triphosphate reuptake inhibitor. Inositol disruptions have been linked to memory impairment and depression.